Question

In carbohydrates, if there is more than 1 asymmetrical carbon atom, how do we determine the D and L-forms?

Answer 1

You look at the stereochemistry of the highest-numbered chiral centre in a Fischer projection.

Explanation:

If the `"OH"` in the highest-numbered chiral carbon (closest to the bottom) is on the right, we have a D-sugar.

If the `"OH"` is on the left, we have an L-sugar.

For example, the Fischer projection of D-glucose is

(from www.chem.wisc.edu)

This is a D-sugar because the `"OH"` on `"C-5"` is on the right.

Do not be fooled into thinking that to get the structure of L-glucose, you just invert the configuration at `"C-5"` to get

(from metabolomics.jp)

This is an L-sugar, but it is L-idose.

To get the structure of L-glucose, you must invert the configuration of every chiral centre.

(from www.easyochem.com)

D-Glucose and L-glucose are enantiomers — nonsuperimposable mirror images of each other.